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Need Residential Retaining Wall Ideas
2

Need Residential Retaining Wall Ideas

Need Residential Retaining Wall Ideas

(OP)
Hello. My property currently has a failing wood-tie multi-tier retaining wall on a slope that needs replacing ASAP. I did much searching and research and have come across many ideas but am looking for guidance on which to go with. COST is a BIG concern, however I want to replace this wall and not have to worry about it ever again...or at least 30 yrs.

Please give me some thoughts...
Current wall to replace is about 1200sqft face total. Three walls each roughly 4 feet high by 100 feet long. They vary in distance from eachother and towards one end, they begin to get closer to eachother then finally curve into eachother and finish off the last 25 feet as one high wall holding back the corner of my side/back yard.

New Ideas were:
1. pumping in concrete to make a single concrete wall about 10 ft high a few feet down the slope from the existing walls, then filling in and grading the property down to this new wall height.

2. three (or two) new tiered modular block walls with geogrid, such as Keystone, Anchor, Unilock or similar.

3. These "Green Walls" "Vegetated Walls" or "GeoWeb" such as the stacked bags or the block that has a large batter that is filled with dirt and plant seeds. This would either be tiered as well or just make one large slope.

I would probably need a fence for the slope idea, or a wall over 5 feet high, so that would need to be factored in to the cost. I did searches and am coming across prices from $20 to $60/sqft.

If someone recommends the Green Wall or Slope idea... I can't seem to find this product in my area?? (I'm in the New York Metro area).
Any help would be greatly appreciated! I am on a budget of about $40k MAX and am hoping for it to be much less than that. Thanks.

Ken
 

RE: Need Residential Retaining Wall Ideas

Can you just install three tiers of 4'high Redi Rock (or similar) walls in front of the three original walls?  Leave the original wall in place.  At only 4' high, the Redi Rock walls should not need geogrids.

www.PeirceEngineering.com

RE: Need Residential Retaining Wall Ideas

(OP)
PEinc,
I looked into something similar, some contractors call it "Mafia Blocks" but I don't think I can get a machine in there to carry these in. Can a bobcat carry these in to the back yard?
Any thoughts on the total $/sqft to use this product?
thanks! Any other suggestions?

RE: Need Residential Retaining Wall Ideas

Those blocks are not very good looking but are cheap in comparison.  They are unreinforced concrete blocks, often 2'x2'x4', made from left over concrete.  Most have raised keyways on top and recessed keyways on the bottom.  You can buy these blocks without the top keyway for the top course of a stacked block wall.  The blocks weigh about 136 to 140 PCF depending on the manufacturer's aggregate.  At 138 PCF, a block would weigh 2208#.  Check what a Bobcat can lift and carry.

Attached as an example is a data sheet for these blocks as available from a Philadelphia area concrete supplier.  
I have designed a few of these wall, usually for temporary use, but I don't have an installed price.  The price would greatly depend on your available access.  It may be easier to set the blocks with a hydraulic crane (picker).

www.PeirceEngineering.com

RE: Need Residential Retaining Wall Ideas

Go with segmented block.  Very nice looking and not all that expensive.  You can do it yourself, also.

RE: Need Residential Retaining Wall Ideas

Installing the geogrids may be more than he wants to do if he needs to remove the existing walls to install the geogrids.

www.PeirceEngineering.com

RE: Need Residential Retaining Wall Ideas

Geogrid is not necessary for a 4' high wall.

RE: Need Residential Retaining Wall Ideas

(OP)
Thanks Ron and PEinc for your input.
Ron, If I did replace the walls with the segmented block at 4' high I will need to do the three tiers again. I was reading up on this stuff and some articles mention "global ??" which I believe has something to do with looking at the whole area to determine the structural requirements. This three-tier wall would only be 4' each one, however it is built on a fairly steep slope... the ground in front of the lowest wall will not be flat approaching the base of the wall. This makes a difference, correct? Also, I read the walls need to be minimum 2x the height stepped back from each other. Geogrid, I believe, may be needed in this case. Any thoughts?



 

RE: Need Residential Retaining Wall Ideas

Three low walls without geogrids should be cheaper to build than one or two taller geogrid walls that could need temporary sheeting to be built.  If you are allowed to move the bottom wall out a little, you may be able to position each of the other two replacement walls so that you have your 2x setback.  Global stability refers to the overall stability of the entire hillside, including the three walls.

Ron is correct that walls with smaller segmental blocks would look nice and are easier to handle, but the design height of a wall also includes the portion of the wall that is buried below the finished grade in front of the wall, usually at least the height of one block.  Therefore, a 4' high wall without geogrids may really be only 3'-4" or less in exposed height.  With bigger blocks, you can go higher without geogrids.  Without geogrids, you could have less excavation (i.e. gouging into the hillside and disturbing the existing walls), less backfill, and probably no temporary sheeting.

www.PeirceEngineering.com

RE: Need Residential Retaining Wall Ideas

PEinc...nice explanation.

RE: Need Residential Retaining Wall Ideas

to accommodate 12 ft of grade change it seems to me you just can't make three 4 ft tall block walls without geotextile reinforcement and say that's good.  I wouldn't do that at all!

any suggestions from this forum seem to make no sense until we see a cross section that extends to either side of this "wall" by a distance of 20 ft or so.  Also, it'd help to know the soil type.

I think if you have a failing timber wall (i.e., one that worked for a while and then "failed"), you could expect a similar design life if you just redid what you have.  Whether you use block or timber would make little difference. If you want a better performance, it'd be good to know what happened. It sounds like you are the victim of failed cohesion and a wall that was stable only when the soil's cohesion was there to assist.  I'm thinking you have some clay on either side of your wall and forming your slope. I'm thinking you need some mechanical stabilization (i.e., geotextile) to reconstruct your wall.

f-d

¡papá gordo ain't no madre flaca!

RE: Need Residential Retaining Wall Ideas

For this particular situation, I wouldn't build a three tier, segmental, small block wall without geogrids.  That's why I suggested the larger blocks, like RediRock.  From my experience, the main problem with timber tie walls, especially in multi-tiered applications, is that they are (were?) usually built by landscape contractors and without any proper design - and they eventually rot.  Rebuilding a properly designed and pressure treated, tiered, timber tie wall or a small-block, tiered, geogrid wall would probably be more expensive than a tiered, big block wall placed in front of the original tie wall.

fattdad is correct.  More information is needed to make a proper decision.  For now, we're just throwing out ideas.

www.PeirceEngineering.com

RE: Need Residential Retaining Wall Ideas

keep in mind that if you go anything higher than 5' high, you will probably need an engineer to submit calculations, stamped for your permit. And frankly, in my opinion a 3-tiered, 12' high wall should do the same thing. Check to see what the permit requirements are.

You are willing to spend $40k and perhaps have a landscaper design it? How long will it last? Will you be willing to spend another $40k in 3 years when it fails?

RE: Need Residential Retaining Wall Ideas

(OP)
cvg,
No way!...I wouldn't leave this up to a landscaper to design. I am a PE and am getting ideas from here and my own research and am going to come up with some type of design and then review with a civil engineer for final review and stamp.

PEinc,
I like your big block in front of the existing wall idea but am questioning this... The bottom wall can be built in front of the old wall, but the second tier would wind up on top of or close to the old first tier wood wall. I'm thinking this old wood will eventually rot and create a weak base for the very heavy new wall above it. I don't know if that will work.

RE: Need Residential Retaining Wall Ideas

In a municipal area I have see something that works, if designed and very cheap.

Materials testing labs readily have to discard tested concrete cylinders.

The original owner of Milwaukee Testing built some nice walls at his house using these cylinders.  The last I knew they still stood.

Where I used to work we had many a homeowner coming by to take our tested cylinders,since we did not break them, once they started to yield.

Otherwise the labs have to dispose of them.

RE: Need Residential Retaining Wall Ideas

That's hard to address without seeing the actual geometry.  However, as you build and backfill the first, lowest wall; you may be able to remove the top foot or two of the lowest, original wall and then fill in its location with well compacted granular material or concrete.  Then repeat for the next tier.

www.PeirceEngineering.com

RE: Need Residential Retaining Wall Ideas

I'd suggest contacting your local Contech sales engineer. They represent a number of different systems.  

RE: Need Residential Retaining Wall Ideas

Generally speaking, all retaining walls over 4' in height are considered "structural retaining walls" and require a structural engineer to certify.  In order to avoid this requirement, and keep costs down, stepped/tiered retaining walls (with 4' max height for each wall) have become popular.  However, without the proper setbacks (2x height is a good example) between the walls, these walls really act like a structural retaining wall.  Lots of landscapers/contractors install these walls without the proper setback, or consideration of soil types and these walls usually fail.   

I'm not a geotech, but I've designed (grading plans for) numerous tiered retaining wall.  From my experience, there's a big difference in retaining wall design if it's for a fill slope or cut slope.  When you have a tiered retaining wall holding up a fill slope (as example, this can be done at the lower portion of someone's front yard, to extend the flat area of yard), you usually do not have to consider reinforcement (geotextile) behind these walls.

But when you "cut" into an existing hill and try to use a tiered retaining wall to hold up this cut slope, geotextile/geogrid is almost always required.  In order to avoid additional excavation for the geogrid, very often soil nails are used for reinforcement.  

I think this may be why you are getting contradicting statements on whether you need reinforcement or not - it depends on if it's holding up a hill, or a flat area of fill.  It sounds like from your description that you are trying to retain a hill.  As previously stated, having a cross section of the existing conditions and soil types would certainly help.  

The use of the large retaining wall blocks could help you avoid reinforcement, but I agree with the construction issues you mentioned above if you attempt to keep the existing walls in place.

I think it's prudent that you are trying to do a lot of this work by yourself (and with some minimal input from this forum).  But, I would recommend that you pay a good local geotech/structural engineer for a few hours of time - you may be surprised how quickly they can determine the most cost-effective design for your situation.  Just make sure you give them some site photos, soil types (if you can dig some holes and take photos), and an existing cross-section.

RE: Need Residential Retaining Wall Ideas

Whether you are holding up a hill or a flat area has nothing to do with the need for geogrids.  The need for grids is based on the wall height, the soil types (retained and internal), grading behind the wall, and global stability.

The discussion of the need for grids in this thread is based on the actual height(s) of the tiers, the available bench widths (1x, 2x, etc.), and the choice of small blocks vs. big blocks.  For tiered wall on an existing hill, installation of grids would most likely be more involved and expensive than no grids.

www.PeirceEngineering.com

RE: Need Residential Retaining Wall Ideas

"Whether you are holding up a hill or a flat area has nothing to do with the need for geogrids."

I understand it does not determine the need for geogrids, but holding up a flat fill or a steep grade directly correlates with two of the design items listed: slope stability and grading behind wall.  So actually, where you are placing the wall has a lot to do with the need for geogrid.

RE: Need Residential Retaining Wall Ideas

(OP)
Yes, this is on a hill.
Thanks for all the responses on this topic. I can't find an engineer in my area! I found one person but he is an over-designer and said he will only consider a reinforced concrete wall that would cost $150k to build... I don't consider someone like this an engineer. Anyone can design a wall that won't fail. An engineer can design a wall that will work yet be cost effective and sensible.
 

RE: Need Residential Retaining Wall Ideas

I stand corrected (sorta).  In my last response, I should have said that whether you are holding up a hill or a flat area is not the only determination for the need of geogrids.  I believe that my second sentence clearly explained my point.  The presence of a sloped surface behind a wall, in itself, is not always a requirement for geogrids.  For example, a very low wall with a slope behind the wall may not need geogrids.  Therefore, all of the factors that I mentioned above are important considerations in determining the need for and design of geogrids.

Two other comments:
1. "But when you "cut" into an existing hill and try to use a tiered retaining wall to hold up this cut slope, geotextile/geogrid is almost always required."  So is temporary sheeting.  Then, you are building two walls - one temporary and one permanent = expensive.
2. "In order to avoid additional excavation for the geogrid, very often soil nails are used for reinforcement."  When this happens, you have not built an MSE wall.  You have built a soil nail wall with an additional segmental block facade.

www.PeirceEngineering.com

RE: Need Residential Retaining Wall Ideas

Dare I suggest using tires infilled? Might be able to get them cheap.  Used in some countries I've worked in. (Naw - probably wouldn't fly in NA) Gabions work - but for 12 ft will need a couple at the bottom - could use the MSE gabion faced walls.

RE: Need Residential Retaining Wall Ideas

Since this is a cut into an existing hill, installing a "small" block retaining wall is not desirable.   For all the reasons mentioned above, this is more than likely not your most cost-effective alternative.  

Gabions are a possibility and desirable if you have access issues.  However, while my past experience with gabion failures could be based on poor construction, I have seen a lot of these walls fall apart over time (bigger rocks tear through facing or smaller rocks get pushed out, creating voids, and settling occurs).  Gabions walls don't necessarily "fail," but can be aesthetically unpleasing.
  
The use for "Redi-Rock" blocks was mentioned above as a possible replacement for the timber wall.  But rather than tiering these walls, have you thought about the possibility of installing one 6'-8' high Redi-Rock wall in front of the first (existing) wall and backfilling behind this wall at a 3:1 or 4:1 slope?  The height of RR wall and slope behind the wall will be based on existing wall batter.  Plan on removing the existing wood timber wall before backfilling.  The cost-effectiveness of this alternative will be greatly dependent on the cost to haul backfill material to your site.  Another positive consideration is that (I believe) Redi-Rock will provide you with a certified ret. wall design (incl. global stability), so you can avoid using your local structural "engineer."

RE: Need Residential Retaining Wall Ideas

Over here in LA you can build a 4' high wall from bottom of footing to top of wall without a permit.  But the dirt behind the wall must be flat.  I am attaching a PDF of a slough wall so you can see what I mean.

A lot of problem projects I have seen have someone who decided to build a retaining wall that holds up a side of the mountain and house at the top, but the footings aren't deep enough to make this wall do anything and no one ever desgined the wall before building.  I would also suggest using someone familiar with the codes in your area to design a cost effective wall.

You should post a picture or sketch of the yard.

Civil Development Group, LLC
Los Angeles Civil Engineering specializing in Hillside Grading
http://www.civildevelopmentgroup.com
http://www.civildevelopmentgroup.com/blog

RE: Need Residential Retaining Wall Ideas

If you're going to go with the redi-rock idea, then the thing to do in my opinion is what civilman mentions.  Do one wall and backfill at 3:1 (or whatever local code dicatates) to tie to existing top of slope.

If you're hard up for fill dirt, pull all the walls out, grade flatter, like 4:1 or so up the hill a bit, then put your taller redi-rock wall in, then go 3:1 to the top of slope.  That way you get some cut down that the bottom that you can use for fill at the top.  Design your wall height, wall location, cut slope, and fill slope around what makes your cut and fill quantities work, so you don't have to bring in or haul off dirt.

In fact, that cut/fill analysis would be a fun exercise in basic civil engineering for you to toy with, xjken.  Google the terms "shrink" and "swell" first.  That should be fun stuff to get you started.

Good luck!
 

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com

RE: Need Residential Retaining Wall Ideas

You can use the geogrids, Before placing geogrids, excavate to the lines and grades shown on the project grading plans and clear the ground surface of all debris or as directed by the
design engineer.what's more the cost of it is very litter, if you make a right products.
And i don't agree with some persons, because the geogrids is not very expensive even if you need the tanser's quality.



gengnan1@gmail.com

RE: Need Residential Retaining Wall Ideas

(OP)
OK all. You have provided much useful info and help. After all this info and reading into this more, I think I came up with a final idea that I am going to submit for permits and construction. I am going to do a 6' segmental block wall with geogrid, gravel, drainage, etc approximately 4ft down from the lowest wall. There will then be a slight slope, maybe 1:5 from that wall to about 15' out where I will then do a second wall of about 3' high using segmental block and no geogrid.

I attached a pic of the existing conditions as someone suggested above.

RE: Need Residential Retaining Wall Ideas


Just goes to show how "permanent" a landscape timber wall really is.  I have one in my yard - the carpenter ants have now overcome whatever cheap pressure treating was used in the timbers.

Retaining walls need to be made of an inert, inorganic material to last.  They get especially difficult to rebuild when those little trees that were planted when it was built have now grown to substantial size.
 

Ralph
Structures Consulting
Northeast USA

RE: Need Residential Retaining Wall Ideas

I have designed and installed litarally millions of square feet of virtually all of the walls described above. Here are some thoughts:

Redi-Rock: big and pretty but expensive to install. 12 feet tall will work without grid with the 60" base IF everything is level on top and bottom. A "bobcat" will have trouble moving these. Installed cost $35-$40 per square feet. They are very proud of their blocks.

Modular block: quick and easy. Design build contractors will handle the design and permits. Installed around $14-$18 per square feet.

Welded wire form slopes: Cheapest. A vegetated slope at 0.33:1 vertical batter will be ideal for your situation and again, a design build contractor will handle this for around $12-16$ installed.

Keep in mind these are for the facing, reinforcement and backfill placement. It will not include the backfill material or demo/excavation. Please don't even use the L word (landscape architect)when it comes to structural retaining walls. I have rebuilt dozens of walls "designed" by landscapers. I would shy away from the reclaimed concrete blocks due to the cold joints and inconsistent construction.

 

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